The invention relates to an endless drive belt for a continuously variable transmission.
A drive belt of this type is generally known, in particular from the patent publication EP-A-0 366 169. The known drive belt comprises a number of transverse elements, each of which is provided with at least one recess which extends between a first and a second main face oriented transversely to the intended direction of movement of the drive belt. The recess offers accommodation for at least one endless carrier and is at least bounded by an abutting face intended for butting against a main face of the carrier and a boundary face oriented transversely thereto. During operation of the drive belt, the abutting face of a transverse element supports a main face of the carrier located on the inside of the carrier. The boundary face of a transverse element delimits the axial freedom of movement of the carrier with respect to the transverse element. The boundary face merges into the first and into the second main face at transition edges. With this arrangement the transition edges are rounded with a certain radius of curvature, such that the boundary face smoothly adjoins the first and the second main face. What is achieved by this means is that physical contact between the carrier and a transverse element is smoothly built up and run down. Thus, when the carrier and the transverse element move relative to one another wear of the drive belt as a consequence of a sharp transition edge is prevented. According to the said patent publication the radius of curvature of a transition edge is preferably greater than, but at least equal to, 0.2 mm.
JP-A-60 127 148 discloses a drive belt, the transverse elements of which are provided with rounded transition edges, the radius of curvature of which is preferably between 0.2 mm and 0.4 mm.
The patent publication JP-A-63 082 842 shows transition elements with which both the said transition edges and the boundary faces themselves are rounded. The radius at which the transition edges are rounded is greater than 0.2 mm.
The aim of the present invention is to provide a drive belt which is improved in qualitative terms and which does away with the apparent prejudice that a relatively large radius of curvature for the transition edges is needed for safe mutual contact between carrier and transverse element. The drive belt according to the invention is disclosed below.
The present invention is based on the insight that the relative movement between carrier and transverse element, where the carrier comes into contact with a boundary face or a transition edge, essentially occurs in situations in which the transfer of force between transverse element and carrier is relatively low. This is the case, in particular, at the location of a section of the carrier which extends in a straight line where the transverse elements are easily able to move in the longitudinal direction and in the axial direction of the carrier. In practice, it is found, surprisingly, that in the situations in which the transverse elements are under heavy load the carrier does not come into contact, or barely comes into contact, with a boundary face of the transverse element. The fact that in such situations the carrier is essentially centred on the abutting face and, as a consequence of this, does not come into contact, or barely comes into contact, with a boundary face or a transition edge contributes to this finding. According to the invention a possible supplementary explanation could also be that modern transmissions are laid out with some misalignment, which is greatest in the lowest transmission ratio of the transmission, or when pulling away, and consequently is relatively small in the highest transmission ratio. Consequently, the most harmful mutual contact between carrier and transverse element would occur only briefly. According to the invention a relatively small radius of curvature is sufficient in order substantially to prevent damage to the carrier and/or a transverse element as a consequence of mutual contact.
One advantage of the drive belt according to the invention is that a transition edge having a relatively small radius of curvature can be produced rapidly and with high accuracy in the material of the transverse element. This benefits the quality of the drive belt, since a drive belt of the type in question comprises a large number of transverse elements, so that even the smallest dimensional deviations between the individual transverse elements can lead to relatively high contact stresses. The high contact stresses have an adverse effect on the fatigue and the wear of the drive belt. Because the transverse elements according to the invention can be produced with high accuracy, the dimensional deviations of the transverse elements are small and a drive belt provided with such transverse elements will continue to function well for a long time.
A further advantage of the drive belt according to the invention is that the said radius of curvature can be applied with the aid of a tumbling process. In the tumbling process milling stones are brought into jolting contact with the transverse elements, as a result of which a rounding is produced over the entire periphery of the transverse element. The rounding of the transition edge in the known construction is usually produced with the aid of a sanding process. A sanding process that can be carried out relatively rapidly is the strip sanding process, in which a transverse element is brought into contact with a rapidly moving sanding strip at the location of a boundary face. Although the tumbling process is a slow process compared with the strip sanding process, for example disclosed in EP-A-0 366 169, a transition edge having a small radius of curvature according to the invention can be produced efficiently by the tumbling process. Advantageous characteristics of the tumbling process compared with a sanding process are the higher dimensional accuracy and improved reproducibility. A reduced spread in the sizing of the transverse elements from which a drive belt is made up benefits the quality of the drive belt. A supplementary advantage of the tumbling process compared with the sanding process is that the said radius of curvature is constant over essentially the entire length of the transition edge because deviations as a consequence of the side edges of a sanding strip curling up are avoided. Such deviations are reflected in a radius of curvature which changes over the length of the edge. Such a non-uniform rounding results in a variation in the local contact pressures between the carrier and a boundary face or a transition edge, as a result of which wear can locally be significantly higher than when the contact between carrier and transverse element is uniformly distributed over the boundary face or the transition edge. With the tumbling process advantageously essentially only profiling in the desired direction is produced.
The drive belt according to the invention also has the advantage that a relatively large proportion of a boundary face of the transverse elements is available for physical contact between carrier and transverse element, so that the contact stresses which arise during use of the drive belt can be lower than in the known construction.
According to the invention a transition edge having a radius of curvature of approximately 0.01 mm already suffices for recognisable reduction in the damage as a consequence of the said contact compared with a non-rounded transition edge. According to the invention, a transition edge having a radius of curvature in the range of 0.08xc2x10.07 mm forms the optimum transition between the boundary face and the first or the second main face.